Electric blasting cap



April 4, 1939. i L. A. 'BURROWS ET AL ELECTRIC BLASTING- CAP Filed Nov. 19, 1956 lllllluln l l lll l lll BY I M ATTORNEY.

Patented Apr. 4,1939

ELECTRIC BLASTING CAP aware Application November 19, 1936, Serial No. 111,580

3 Claims.

The present invention relates to new and improved electric blasting initia'tors and improved methods of blasting therewith, particularly in the art of seismographic exporation.

v It is well known that in seismographic exploration for mineral and other deposits, fundamental calculations are based upon the time interal within which vibrations from a given explosion are transmitted for a known distance through the earth. Originally this time interval was calculated by noting the instant at which the current was applied to the firing circuit of an electric blasting cap by means of which the explosion was to be initiated, and determining the lapse of time which occurred before the vibration arrived at its destination. This procedure gave an inaccurate result due to the fact that the electric blasting cap common to the art does not fire immediately upon application of cur- 99 rent thereto. An appreciable time lag occurs between the application of current and the detonation.

In order to avoid the foregoing difliculty, a system was adopted wherein the time interval 35 was measured from the instant at which the firing circuit of the cap was broken. The results obtained were more nearly accurate than before, but there was still the difliculty that detonation seldom occurred at exactly the instant at which the bridge wire was broken. The bridge wire frequently fused through before the ignition mixture was fired, causing an appreciable time lag or induction period between the instant at which thecircuit was broken and the instant of detonaation. In cases wherein the seismographic measurements involved a short time interval, the foregoing time lag was still large enough in proportion to introduce appreciable error into the the determinations.

It has been proposed to. solve the foregoing problem by the use of such a fast ignition composition that no appreciable induction period will be present. Such a solution is disclosed and claimed inthe copending application of L. A.

Burrows, Serial No. 47,382, filed October 30, 1935.

An additional difliculty is introduced by the use of such fast ignition mixtures. Such compositions are normally highly sensitive to static electricity so that elaborate precautions must be taken in the construction and use of the caps containing them.

The chief object of the present invention is the development of a new and improved electric blasting initiator for seismographic-determinations and the like. A further object is a new and improved method of blasting for attaining accurate results in sesimographic determinations. An additional object is an improved electric blasting initiator so constructed that the bridge wire fails to fuse before detonation.

Further objects will be apparent from the following detailed description of our invention.

The foregoing objects are readily achieved, according to the principles of our invention, by constructing an electric blasting cap providedwith a bridge wire so characterized that it fails to fuse upon the passage therethrough of a quantity of electricity sufiicient to efliciently detonate the cap. I

In order to describe our invention more clearly, we shall refer to the attached drawing which represents several embodiments thereof. It is to be understood, however, that this is done solely by way of illustration, and is not to be regarded as a limitation upon the scope of our invention which has many important applications other than those herein particularly illustrated.

Referring generally to the drawing, Figures 1, 2, and 3 represent respectively vertical sections of a bridge plug type, a concave plug type, and a match-head type of electric blasting cap in accordance with our invention. Similar characters are used throughout the several views and in the accompanying description, to indicate corresponding parts.

Referring particularly to Figure 1, the blasting cap illustrated comprises a capsule or shell I of metal or like rigid material. In the base of said shell is placed a base charge 2, and an ignition composition 4. The latter surrounds the bridge wire 8, attached to the ends of the leg wires 6 and I, held in fixed space relationship by means of the bridge plug 5. The bridge wire 8 is so constructed that it will fail to fuse upon the passage therethrough of any given charge of electricity selected for its capacity to eiliciently fire the cap. The cap is closed by means of the waterproofing composition 9 and the sulfur seal Ill.

In Figure 2, the shell I contains the base charge 2, suitable priming charge 3 and the concave plug 5 charged witha suitable ignition composition 4 around the bridge wire 8. The bridge wire is constructed according to the principles of our invention so that it does not fuse but is ruptured by the detonation of the cap.

In Figure 3, the construction is varied by the introduction of the match-head structure ll, covered by the ignition composition, 4, and disposed within the paper cylinder I2. The matchof 80% nickel and 20% chromium, having a head ll comprises two metal plates or foils separated by a strip of insulating material l3. One leg wire is connected to each plate and the bridge wire 8 forms the connection between said foils.

for an instant only, will not do so.

The bridge wire is constructed according to the principles of our invention so that it is ruptured by detonation of the cap and is not fused.

In constructing an electric blasting initiator having a. bridge wire according to the foregoing principles, it is important to note that we are interested in such a wire which will not fuse upon the passage of electricity sumcient to fire said initiator efliciently. In determining the magnitude of such a quantity of electrical energy, it should be remarked that the size of the current in amperes, passing through the bridge wire, is not of sole importance. "A small current passed through a given wire for a given time may melt the wire, whereas a much larger current applied Our invention embraces the concept that for any initiator assembly there may be determined the quantity of electricity necessary to efficiently cause detonation,- and a bridge wire may be employed which will not fuse upon the passage therethrough of said quantityof electricity, whatever the source may be.

A more specific application of the invention may be made with reference to a particular source of electrical energy. In the field of seismographic exploration, it is common to fire electric blasting caps by means of an electric blasting machine.

An electric blasting machine of the type commonly known to the art comprises a manually operated electric dynamo or generator. Upon moving the handle or lever thereon, a discharge of electricity passes through any circuit connected therewith. Since said discharge does not last long enough to overcome the inertia in either an ammeter or a voltmeter, the nature and quan-' tity of the discharge is best determined by means of the well known cathode-ray oscillograph. Due to the inertialess character of the electron beam, the peak voltage and amperage available may be readily determined, in spite of the short .duration of the discharge. Once the discharge characteristics of a given machine have been determined, a suitable bridge wire may be constructed of such material and thickness that it will not fuse upon the passage of the particular discharge therethrough. A cap equipped with such a bridge wire will be highly desirable for use in seismographic determinations, on conjunction with the particular type of blasting machine or one which delivers a'similar discharge, since the firing cir=- cuit will not be broken till the detonation of the cap. An illustration of such an application of our invention is disclosed in the following example.

Example A particular electric blasting machine was selected and the discharge characteristics were measured by means of the cathode ray oscillograph. In measuring the peak amperage, resistances from one up to and including five ohms times the rated capacity of the machine were connected across the binding posts. The machine was found to generate an instantaneous peak amperage of about 15.0, which amperage dropped to approximately zero in 0.3 second as the discharge was completed. The maximum voltage was determined to be 225. Bearing in mind these values, it was discovered, by trial and error, that abridge wire constructed of an alloy diameter of 0.005 inch, could not be fused by the passage of a discharge from the blasting machine in question, yet was heated thereby to such an extent as to fire any of the ignition mixtures common to the art. It was then determined that other machines of the same design delivered almost identical discharges and that all such machines successfully fired electric blasting caps provided with the bridge wires constructed according to the foregoing specifications, and in all cases the principle of the invention was operable to such an extent that in no instance did the. bridge wires fuse. The firing circuit was broken by detonation in every case. As a matter of standard, it was determined that all the commercial blasting machines tested produced a discharge capable of fusing a bridge wire constructed of 80% nickel and 20% chromium alloy and having a diameter of 0.00225 inch. Yet all failed to fuse the wire of our invention.

The foregoing example is merely a single embodiment of our invention by way of illustration. The broad principles of the invention may be applied to develop a bridge wire adaptable to function without fusing with any known source of electricity, whether derived from the discharge of an electric blasting machine, battery, or any controlled current source, the only precaution being that the caps so provided be fired by electrical sources known to be compatible therewith without fusing the bridge wires.

Although we have described our invention in detail and have therefore utilized certain specific terms and language therein, it is to be understood that the present disclosure is illustrative rather than restrictive and that changes and modifications may be resorted to without departing from the spirit or the scope of the claims appended hereto.

We claim 1. The method for the reduction of the inherent time lag in seismographic blasting between the breaking of the firing circuit and the detonation of the explosive charge, by the elimination of that portion of said time lag due to premature rupture of the bridge wire of the electric blasting cap; which comprises controlling the electric current to said electric blasting cap so that the bridge wire of said electric blasting cap will be heated to a sufllicient temperature to detonate said cap but insufflcient to cause fusion of said bridge wire, whereby said electric circuit will be disrupted substantially concurrently with the detonation of said blasting cap and thus more nearly coincident with the detonation of said explosive charge.

2. The combination in a seismographic blasting system of the type including an electric blasting cap, a timing circuit and a source of current; which comprises an electric blasting machine as the source of current constructed to deliver a peak'amperage of no more than 15.0 diminishing to approximately zero in 0.3 second, a conventional seismographic timing circuit and an electric blasting cap, the bridge wire of which serves as an element of said timing circuit, said bridge wire being constructed of an alloy of approximately 80% nickel and 20% chromium having a diameter of at least approximately 0.005 inch, said electric blasting cap being connected in firing recurrently with the detonation of said blasting cap and more nearly coincident with the detonation of said blasting charge.

3. The combination in a seismographic blasting system of the type including an electric blasting cap, a timing circuit and a source of current; which comprises an electric blasting machine as the source of current, a conventional seismographic timing circuit and an electric blasting cap the bridge wire of which serves as an element oi said timing circuit, said bridge wire being selected for its predetermined resistance to fusion, said 10 electric blasting machine being connected in firing relationship with said electric blasting cap and being constructed to deliver an amount of current lnsuflicient to fuse said bridge wire, whereby said timing circuit will be disrupted concurrently with the detonation of said blasting cap and more nearly coincident with the detonation of said blasting charge.

LAWTON A. BURROWS. STANLEY L. HANDFORTH. 

